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Error 121

SolarEdge Solar Inverter

Severity:

Source: OnlyErrorCodes.com — Every error code. One place.

SolarEdge Error 121 — Isolation Check Fail

What Does This Error Mean?

Error 121 on a SolarEdge inverter means the isolation (insulation) test has failed at startup — DC wiring is leaking to ground beyond safe limits. The inverter blocks production until the leak is found and fixed. Cause is almost always rooftop water ingress or damaged DC cable insulation.

Affected Models

  • SolarEdge SE3000H
  • SolarEdge SE7600H
  • SolarEdge SE10000H
  • SolarEdge SE11400H-US
  • SolarEdge HD-Wave

Common Causes

  • Water ingress at a junction box, MC4 connector, or panel back contact
  • Damaged DC cable insulation (rodent bite, abrasion, UV degradation)
  • Failed module backsheet allowing internal moisture
  • Improperly tightened MC4 connector allowing humidity ingress
  • Failed power optimizer allowing ground fault

How to Fix It

  1. Check the weather correlation.

    Error 121 often appears the morning after rain. Note in SolarEdge monitoring whether Error 121 events correlate with weather. Wet-conditions Error 121 strongly suggests water ingress at a junction or connector.

  2. Wait for dry conditions.

    Some Error 121 events clear on their own when the wet area dries. The inverter may auto-restart in dry weather. Use this as confirmation that water ingress is the cause.

  3. Contact installer for full diagnosis.

    An installer with an IR meter can locate which string and which area has the isolation fault. They may need to test individual MC4 connectors and rejoin any with water ingress.

  4. Replace damaged components if found.

    MC4 connectors with water ingress need replacement. Damaged cable sections need cutting back and reterminating. A failed module needs replacement. Cost varies USD 200–800 depending on what is found.

  5. Add monitoring after repair.

    Ask the installer to log a baseline isolation reading after repair. Future deviation from this baseline catches new issues early — before they cause production loss.

When to Call a Professional

Error 121 requires rooftop work and DC isolation testing — should only be done by a qualified solar installer with PV training and appropriate testing equipment.

Frequently Asked Questions

Why does Error 121 only happen in the rain?

Wet weather opens up tiny leakage paths to ground via water bridges. Dry weather closes them. If your Error 121 is rain-correlated, the cause is definitely water ingress at a connector, junction, or panel. Find and fix the entry point.

Is Error 121 a safety issue?

Yes — sustained DC ground leakage is an electrical safety risk and is regulated by every grid code. The inverter's protection is correct to block production. Do not bypass — fix the underlying isolation fault.